Piezoelectric pressure sensor

ABSTRACT

A pressure sensor comprising a piezoelectric material formed in a geometry which provides a piezoelectric output signal when subjected to a pressure. The piezoelectric material also has a pyroelectric output signal. The present sensor further comprises conductive contacts having a geometry for enhancing the piezoelectric output signal and for substantially eliminating the pyroelectric output signal.

BACKGROUND OF THE INVENTION

The present invention is a piezoelectric pressure sensor comprising amaterial having both a piezoelectric and pyroelectric output signal. Thesensor further comprises electrical contacts having a geometry forenhancing the piezoelectric output signal of the sensor and forsubstantially eliminating the pyroelectric output signal. Although thesensor is disclosed as a zinc oxide (ZnO) on silicon diaphragmstructure, the invention is not limited to such a structure.

Silicon diaphragms have been used in a variety of pressure sensordevices such as capacitive pressure sensors and piezoresistive pressuresensors; see, for example, C. S. Sander, J. W. Knutti, J. D. Meindl,IEEE Trans. Ed. 27, No. 5, May 1980. The use of silicon as part of asensing element is especially attractive since it allows the silicondiaphragm and the electronic part of the device to be integrated on thesame silicon substrate. Such a structure increases the accuracy and thestability of the device and lowers its cost. Further, etching techniquesenable the making of thin diaphragms which increase the pressuresensitivity of such devices.

ZnO is a piezoelectric material which can be sputtered on a variety ofsubstrates including silicon and metals, and ZnO can be used as theactive element of such a pressure sensor; see, for example, P. L. Chen,R. S. Muller, R. M. White, R. Jolly, IEEE 1980 Ultrasonic Symposium.

When a silicon substrate on such a device is flexed, a stress patternappears in the ZnO layer and induces a piezoelectric polarizationparallel to the 3 axis of the ZnO lattice, which is perpendicular to thesurface of the layer. A voltage output can be then collected across acapacitor formed by two electrodes deposited at the top and the bottomsurfaces of the ZnO layer. Such a capacitive arrangement, however,typically presents the disadvantage of having to connect the topelectrode to the electronics on the silicon substrate, thus involving astep coverage problem, especially in the case of thick ZnO layers on theorder of 10 microns in thickness.

The other problem involved in using ZnO as the active element arisesfrom the pyroelectric properties of ZnO, which produces a voltage outputdue to temperature variations.

The present sensor comprises an electrode configuration which not onlysolves the two problems mentioned above but also enhances thepiezoelectric output signal from the sensor. Thus, the electrodeconfiguration of the present sensor has three technical advantages overprior art piezoelectric pressure sensors. The electrode configuration ofthe present sensor eliminates the need to connect the top capacitativeelectrode to the electronics on the semiconductor substrate, thuseliminating a step coverage problem. In addition, the electrodeconfiguration of the present sensor substantially eliminates thepyroelectric voltage output due to temperature variations. Further, theelectrode configuration of the present sensor enhances the piezoelectricoutput signal generated by the piezoelectric sensing material in thesensor.

SUMMARY OF THE INVENTION

The present invention is a pressure sensor comprising a piezoelectricmaterial formed in a geometry which provides a piezoelectric outputsignal when subjected to a pressure. The piezoelectric material also hasa pyroelectric output signal. The present sensor further comprisesconductive contacts having a geometry for enhancing the piezoelectricoutput signal and for substantially eliminating the pyroelectric outputsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, and 3 illustrate the structure and electrode configurationof the present sensor; and

FIGS. 4, 5, and 6 relate to operation of the present sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present sensor comprises a siliconsemiconductor body 10 having a diaphragm 12 etched into the body, thediaphragm having a diameter 14. Diaphragm 12 may be etched into body 10through standard isotropic electrochemical etching from surface 16.Semiconductor or substrate 10 is typically on the order of 250 micronsthick.

The present sensor further comprises a layer 18 of dielectric such assilicon dioxide, dielectric layer 18 being formed on a surface 20 ofsubstrate 10. For the preferred embodiment illustrated, the dielectriclayer is on the order of one micron in thickness.

A pair of electrodes comprising an outer ring electrode 22 and a centerelectrode 24 are formed on a surface 46 of dielectric layer 18. In theembodiment illustrated, electrodes 22 and 24 are concentric and are ofapproximately equal surface area. In other embodiments of the presentinvention, electrodes 22 and 24 would not have to be round orconcentric. For example, they could be of a square configuration.Electrodes 22 and 24 comprise leads 26 and 28, respectively, leading tosupport electronics 30, which are typically integrated on substrate 10.

The present sensor further comprises a layer 32 of dielectric such assilicon dioxide formed over contacts 22 and 24 and dielectric layer 18.

A layer 34 of piezoelectric material is typically formed abovedielectric layer 32. In the preferred embodiment of the present sensor,layer 34 comprises ZnO.

Layer 34 is typically covered with a dielectric layer 36 on top of whichis typically formed an electrode 38. In the embodiment shown, electrode38 is round and has approximately the same diameter as diaphragm 12 andthe outer diameter of outer ring electrode 22. As indicated with respectto electrodes 22 and 24, electrode 38 would not have to be round.

A layer 40 of dielectric such as silicon dioxide is typically formedabove electrode 38 and dielectric layer 36.

Dielectric layers 32 and 36 isolate piezoelectric layer 34 fromelectrodes 22, 24, and 38 and, together with dielectric layers 18 and40, encapsulate the electrodes and leads 26 and 28. Layers 32 and 36effectively reduce charge leakage into piezoelectric layer 34 and,therefore, effectively retards the cancellation of piezoelectric chargein layer 34.

The present sensor illustrated is typically fabricated by selecting asilicon substrate 10 having proper crystallographic orientation.Integrated support electronics 30 are implanted or otherwise fabricatedonto substrate 10. Substrate 10 is then covered with layer 18 of silicondioxide. As previously indicated, layer 18 is typically on the order ofone micron in thickness. A layer of aluminum typically on the order of0.5 micron in thickness is then deposited above layer 18, and thealuminum is selectively etched to form electrodes 22 and 24. Layer 32 ofsilicon dioxide is then sputtered down over contacts 22 and 24 and layer18. The thickness of layer 32 above electrodes 22 and 24 is typically onthe order of 0.2 micron in thickness. Layer 34 of ZnO is thensputter-deposited above layer 32, layer 34 typically being in the rangeof approximately 3-10 microns in thickness. Layer 36 of dielectric isthen sputter-deposited above layer 34, layer 36 typically being 0.2micron in thickness. A layer of aluminum, typically 0.5 micron inthickness, is then sputter-deposited above layer 36. Aluminum layer 38is then selectively etched to define electrode 38. Layer 40 of silicondioxide is then sputter-deposited above electrode 38 and dielectriclayer 36, layer 40 typically having a thickness above electrode 38 of0.2 micron. Layers 40, 36, and 34 are then selectively etched to sizeand the edges of ZnO layer 34 are sputter-deposited with silicondioxide. Diaphragm 12 is then selectively etched from surface 16 ofsemiconductor body 10 through isotropic electrochemical etchingtechniques.

The present sensor comprises two ZnO piezoelectric capacitors C1 and C2,which are schematically illustrated in FIGS. 4 and 6. One of thecapacitors C1 and C2 is formed by center electrode 24 and electrode 38,and the other of these capacitors is formed by outer ring electrode 22and electrode 38. The two capacitors are connected in series throughelectrode 38. Connections to electronics 30 integrated within siliconsubstrate 10 are made only with the pair of electrodes 22 and 24 vialeads 26 and 28 respectively. As previously indicated, this provides asubstantial advantage to the present sensor since no connections need tobe made to upper electrode 38, thus avoiding a step coverage problem intrying to made leads which would connect to upper electrode 38.

ZnO is not only a piezoelectric material, but also has pyroelectricproperties. The pyroelectrically induced polarization is perpendicularto the ZnO layer surface and can be assumed to be uniform in all oflayer 34. The pyroelectric polarizations induced in capacitors C1 and C2are shown in FIG. 4. By properly choosing the electrode areas, thepyroelectric outputs of the two capacitors C1 and C2 can be made tocancel each other out. Roughly, this means that, if edge effects of thecapacitors are neglected, center electrode 24 and outer ring electrode22 are of equal surface area.

When diaphragm 12 is flexed under a uniform pressure, the stress patternthat develops in the ZnO layer is shown in FIG. 5. When the pressure isapplied from the layer 34 side of diaphragm 12, the central part oflayer 34 is in compression while the outer part of layer 34 is inextension.

The piezoelectric polarizations induced in capacitors C1 and C2 areshown in FIG. 6. The piezoelectric signals add to one another in thepresent sensor, thus increasing the pressure sensitivity of the deviceover sensors in the prior art. The piezoelectric output can be computedby integrating the local piezoelectric signal over the electrode area,and the electrode dimensions can be optimized to obtain a maximumpiezoelectric response while satisfying the equal area conditionnecessary to the cancellation of the pyroelectric responses.

In a preferred embodiment of the present sensor, electrode 38, diaphragm12, and the outer diameter of outer ring electrode 22 had a diameter of3.18 millimeters. The inner diameter of outer electrode 22 was 2.38millimeters, and the diameter of central electrode 24 was 2.1millimeters. Diaphragm 12 had a thickness between surface 42 and surface20 of approximately 30 microns.

The present invention is to be limited only in accordance with the scopeof the appended claims, since others skilled in the art may devise otherembodiments still within the limits of the claims. For example, althoughtypical construction and dimensions have been disclosed, the presentsensor is not limited to such construction and dimensions.

The embodiments of the invention in which an exclusive properly or rightis claimed are defined as follows:
 1. A pressure sensorcomprising:piezoelectric means comprising a piezoelectric material forproviding a piezoelectric output signal when subjected to a pressure andfor providing an undesirable pyroelectric output signal, thepiezoelectric means having first and second surfaces; and electrodemeans comprising a first contact covering at least a portion of thefirst surface, the electrode means further comprising a pair of contactscovering a portion of the second surface, the contacts having a geometryfor enhancing the piezoelectric output signal and for substantiallyeliminating the pyroelectric output signal.
 2. The sensor of claim 1wherein the pair of contacts have a concentric geometry.
 3. The sensorof claim 2 wherein the pair of contacts have substantially equal surfaceareas.
 4. The sensor of claim 3 wherein:the electrode means comprises adiaphragm of semiconductor material etched out of a semiconductor body;and the pair of contacts are located between the piezoelectric means andthe semiconductor body.
 5. The sensor of claim 4 wherein thepiezoelectric material comprises ZnO.
 6. The sensor of claim 1 whereinthe pair of contacts have substantially equal surface areas.
 7. Thesensor of claim 6 wherein:the electrode means comprises a diaphragm ofsemiconductor material etched out of a semiconductor body; and the pairof contacts are located between the piezoelectric means and thesemiconductor body.
 8. The sensor of claim 7 wherein the piezoelectricmaterial comprises ZnO.
 9. The sensor of claim 1 wherein:the electrodemeans comprises a diaphragm of semiconductor material etched out of asemiconductor body; and the pair of contacts are located between thepiezoelectric means and the semiconductor body.
 10. The sensor of claim9 wherein the piezoelectric material comprises ZnO.
 11. The sensor ofclaim 1 wherein the piezoelectric material comprises ZnO.